The present invention pertains to a polyamide powder formed by porous elementary particles having a "gypsum rose" structure. The particularity of these particles is that they have a high absorption capacity due to the high number of pores and the large pore volume. These properties of the particles consequently lead to a polyamide powder of a large specific surface and low apparent density.
The polyamide powder is prepared by anionic polymerization of lactam in a solvent medium in the presence of at least one alkylene amide. The structural particularity of the particles is achieved by initiating the polymerization in a solvent medium which is supersaturated with lactam at the initiation temperature.
A process for manufacturing a polyamide powder by anionic solution polymerization of lactam in the presence of an alkylene bisamide is described in French Patent No. 2,576,602. According to the technique disclosed in this patent, which consists of dissolving the total amount of the lactam in the solvent in the presence of an alkylene bisamide prior to the polymerization initiation, powders with controlled particle size composition and controlled molecular weight are obtained. The particles obtain have a small specific surface (below 9 m.sup.2 /g) and consequently a very low porosity.